Optical systems of the above type are used in a large number of devices, for example handheld devices with imaging capability, such as handheld scanners and optical pens. In such devices, it is important for the optical system and the optical components included to be made as compact as possible so as to keep the total size down. For an optical system used in an optical pen, the relevant object has usually an extent of less than one centimeter, and the image formed by the imaging optics is usually within a surface of a few square millimeters. Such an optical pen is disclosed in WO 01/71654, which is incorporated herein by reference. Of course, other dimensions of the optical system may be involved for other types of devices.
The optical system typically includes imaging optics, a radiation source and a radiation sensor. For reasons of production, it is important that the components included in the optical system can be mounted in a manner which is as simple, robust and space-efficient as possible. In handheld devices, it is common to mount the imaging optics and the sensor side by side with the radiation source, which results in separate illumination and imaging paths. Such mounting, however, requires a great deal of space in the handheld device.
A typical requirement placed on optical systems in handheld devices is that they must be able to adequately image objects at different distances. This means that the system should have a sufficiently large depth of field, which calls for a high f-number of the system and, thus, a relatively small aperture stop. Such a system will permit relatively little radiation to reach the image plane from the illuminated object plane. The object plane must therefore be exposed to a high intensity of illuminating radiation.
Further, in optical systems with separate illumination and imaging paths, the illuminated area and the imaged area in the object plane can only be brought to coincide for a given nominal object distance. Whenever there is a need to accommodate for various object distances within the depth of field, it is necessary to illuminate a considerably larger object plane area than the one that is to be imaged at the nominal object distance. Thus, the radiation from the radiation source is used inefficiently, which further accentuates the need for a powerful radiation source.
Such a powerful radiation source may exhibit several drawbacks, such as being expensive, non-durable, power-consuming and bulky.
The above problems may also be present in optical systems with partly overlapping illumination and imaging paths. WO 00/72287 discloses an optical pen with such an optical system, in which a beam splitter is arranged to reflect radiation from a radiation source onto an object plane and to transmit any retro-reflected radiation from the object plane to imaging optics that forms an image of the illuminated object plane on a sensor. Such a system makes inefficient use of the available radiation, since every interaction with the beam splitter results in a considerable loss of radiation. Typically 50% of the incident radiation from the radiation source is transmitted by the beam splitter instead of being reflected towards the object plane, and typically another 50% of the retro-reflected radiation is reflected by the beam splitter instead of being transmitted to the sensor. Further, any such transmission of the incident radiation from the presumably powerful radiation source is likely to produce significant background radiation which might interfere with the transmitted retro-reflected radiation that forms the image signal on the sensor. Additionally, the beam splitter is a potentially expensive component, in particular if it should exhibit controlled surface and transmission properties.
A similar optical arrangement is disclosed in GB-A-2 166 831.
The prior art also comprises U.S. Pat. No. 6,114,712 which discloses a handheld scanner with yet another form of optical system with partly overlapping illumination and imaging paths.